The present invention relates to vibration isolation apparatus included in means for supporting and securing a first member to a second member. There are many commercial environments where a first member needs to be supported by a second member, with at least one of the members being vibrated such that it is desirable to attenuate the transmission of the vibration induced forces therebetween. Attenuation of the vibration induced force transmission minimizes structural wear on the respective members and reduces undesirable noises generated by such vibrations.
One particular environment contemplated by the present invention is the environment of internal combustion engine supports in vehicles, so-called engine mounts. The invention also contemplates active vibration isolation mounting arrangements in environments involving vibrating machinery that must be supported, including compressors, generating plants, machine tools, and the like.
Almost since the advent of the use of internal combustion reciprocating piston engines to power automotive vehicles, some type of elastic cushioning mount has been utilized to attenuate the transfer of vibrations between the vehicle frame and the engine. Typically, such engine mounts include rubber cushions or bushings situated between clamping flanges that are bolted to the vehicle frame and engine brackets to hold the engine in place. These elastic cushioning mounts can be generally referred to as "passive" mounts in that they tend to attenuate the transfer of vibration forces by passively generating reactive forces resisting relative movements of the parts. FIG. 1, described below, schematically depicts a prior art passive engine mount arrangement.
In the last five years, there have been suggestions to utilize so-called "active" vibration isolation mounts for supporting vehicle engines by a vehicle frame. Two SAE Technical Papers presented at the International Congress and Exposition, Detroit, Michigan, Feb. 29-Mar. 4, 1988 relate to such mounts--Paper 880074, titled: ACTIVE FRAME VIBRATION CONTROL FOR AUTOMOTIVE VEHICLES WITH HYDRAULIC ENGINE MOUNTS, by P. L. Graf et al and Paper 880075 titled: OPEN-LOOP VERSUS CLOSED-LOOP CONTROL FOR HYDRAULIC ENGINE MOUNTS, by R. Shoureshi et al. The active vibration isolation mounts referred to in the above-noted SAE Technical Papers 880074 and 880075 provide for the generating and application of counter, opposite vibration forces responsive to the vibration experienced. In these papers, a hydraulic actuator with fluid pressure chambers at opposite sides of a diaphragm is proposed as an active vibration isolation mount. A support bushing is carried by the diaphragm. This support bushing in turn is connected to apply counter forces to a member being vibrated in response to changes in actuating fluid pressure applied to the fluid chambers. FIG. 3, described below, schematically depicts such a prior art active engine mount arrangement.
The active vibration isolation mounts such as mentioned above are useful in that they effectively reduce the transmission of the vibration force movements between the elements, and thus minimize the need for passive supports and provide for the response flexibility that a microprocessing unit can provide to adjust to changes in vibration frequency and amplitude patterns.
However, a problem with some of these prior art active vibration isolation systems is that a failure of the actuator may result in a "hard" short between the bottom of the mount and the vibration disturbance. Further, arranging the actuator in series with the force transfer between the respective vibrating members requires an extremely sturdy actuator construction to accommodate the forces involved.
An example of an actuator for an active vibration isolation system is a conventional hydraulic cylinder. Such a cylinder can fail, for example, by having the relatively movable metal parts freeze-up (stick). In such a case, the hydraulic cylinder becomes a "hard" short to vibration forces between the respective parts to which it is connected. An example of a hydraulic actuator can be found in an article entitled, "Counter Vibrations Smooth Copter Ride", by Tom Shelley, published May, 1988. In that article an engine mounting arrangement for a helicopter engine is described including a hydraulic actuator for an active vibration cancellation system mounted concentrically within an elastomeric passive mount. In the event that the hydraulic actuator (a hydraulic cylinder) fails, a "hard" short will form between the helicopter engine and its frame.
An object of the present invention, therefore, is to design an active vibration isolation mount which avoids the problem of such a "hard" short.
Another object of the present invention is to provide for a sufficiently strong mount that can be operated in an active mode while remaining out of the static load path between relatively vibrating members.